Toner image fixing devices used in electro-photographic type copying machines, facsimiles, printers and other instruments using electro-photographic process are generally of the thermal fixing type that fixes toner image on a recording medium by fusing. A fixing portion is composed of a Eli; fixing roller and pressure roller pressing the fixing roller. Either one or both rollers are heated. While the recording member passes a nip between the two rollers, a toner image formed on the recording member is fixed thereon by the effect of heat and pressure.
The fixing roller is a thin-wall aluminum cylinder coated with well-releasable and heat-resistant synthetic resin, e.g., fluorocarbon resin (PFA, PTFE), and includes a heater lamp inserted in its center bore. The fixing roller is rotatably supported on a roller supporting member.
The pressure roller is a metal roller coated with silicone rubber and is rotatably supported at both ends by a pair of pressure-roller supporting members. The pressure roller is pressed against the fixing roller at a constant force of compression coiled springs.
The surface of the fixing roller is heated by the heater lamp mounted therein. Surface temperature of the fixing roller is controlled by a temperature adjusting circuit according to a signal from a roller-surface-temperature sensing means disposed near the fixing roller. The roller-surface-temperature sensing means comprises a temperature sensing element (e.g., thermistor) pressed against the surface of the fixing roller to minimize a disturbance.
While the recording medium carrying a toner image formed thereon passes through a nip formed between the fixing roller and the pressure roller, the toner image is heated and fixed by fusing onto the recording member. Thus, the fixing portion has heating means for heating and keeping the roller surface at a constant temperature to fuse toner on the recording medium. Accordingly, the temperature of the roller surface may abnormally rise if the roller-surface temperature control malfunctions due to abnormal operation of a main machine. The machine is provided with a safety device that may prevent occurrence of smoke and fire in the machine in the worst case.
The safety device may be composed of a separate temperature sensing means and a separate control circuit or the temperature sensing means inserted in series in the heater lamp circuit. The safety device detects by the temperature sensing means that a temperature of the roller exceeds a specified value. It acts upon the control circuit to stop power supply to the heater lamp or directly switches off the heater lamp circuit. A temperature sensing element (e.g., a thermostat, a temperature fuse and a thermal protector) is usually used as means for sensing an abnormally rising temperature. This temperature sensing element is usually disposed apart from the roller since the roller surface temperature rises gradually.
Recently, electro-energy saving of electric appliances has become a very important problem from the environmental view point. Electrophotographic type printers and other machines that use electrophotographic process are also required to be of electro-energy-saving type. A most energy consuming portion of an electrophotographic machine (printer) is a toner-image fixing device that consumes a large part of a total electric power consumption of the machine. The reduction of power consumption of the fixing device is an essential object of the electrophotographic machine.
It is, however, very difficult to reduce the power consumption of the conventional fixing device without changing its construction. The reason is as follows:
Once switched on, the conventional fixing device keeps its roller at a constant temperature even while print is not needed. Preheating of the roller is necessary for printing without waiting time.
It is therefore possible to reduce power consumption of the fixing device by switching off the heater except for printing time. However, this solution encounters a new problem that every time before printing it is required to wait until the roller gets a temperature necessary for fixing toner image.
Methods which are thought effective to solve this new problem "increased waiting time" are: (1) increasing wattage of a heater lamp and (2) reducing a diameter or wall thickness of a fixing roller. The method (1) is simple but not practical because waiting time can be shortened but electric power consumption is increased. The method (2) is to make the roller be quickly heated up by reducing its heat capacity and thereby shorten the waiting time. This method is also simple but involves the following problem:
The fixing roller of a reduced heat-capacity can be so rapidly heated up that the conventional temperature sensing element can not response to a change of surface temperature of the roller. This means that abnormal temperature rising may not immediately detect, causing smoke and fire in the machine. So, practical use of an energy-saving fixing device of a small heat capacity has not been realized because the safety operation can not be guaranteed.
On the other hand, response of temperature sensing elements have been improved. Japanese Laid-Open Patent Publication No. 58-118681 discloses such a method that a temperature sensing element is disposed apart from a roller and a heat-reflector is disposed behind the temperature sensing element to increase a sensing surface temperature of the element. Japanese Laid-Open Patent Publication No. 63-169680 discloses a temperature sensing element covered with a heat-conductive member, which butts upon a roller.
The above-mentioned methods can be effectively applied to the conventional fixing unit which temperature rises moderately but can not be applied to a small-heat-capacity type fixing device that may realize considerable reduction of electric power consumption. For example, the art described in Japanese Laid-Open Patent Publication No. 58-118681 can not realize the small-heat-capacity type fixing unit because a temperature sensing portion of a safety device having no contact with a fixing roller may cause a large disturbance and has an insufficient response resulted from low heat-conductivity. The prior art disclosed in Japanese Laid-Open Patent Publication No. 63-169680 can not be applied to a small-heat-capacity type fixing unit because the temperature sensing element covered with aluminum block has a worse response due to large heat transfer and the temperature sensing element covered with teflon has a worse response due to low heat-conductivity.